With a direct-fired steam generator, one can directly control the amount of water entering the system, and can indirectly control the amount of heat generated by the system by varying either air flow and/or fuel flow. It is desirable to have a constant steam temperature produced by the steam generator, which, in the present embodiment that is used for re-hydrating crop just prior to baling, has a continuously variable heat output from about 1.5-4.0 million BTU/hr.
Steam temperature differential, i.e., the difference between the present steam temperature and the desired steam temperature, does not provide enough information to control the steam temperature within the desired response time. At different heat outputs, different amounts of water are needed to get the same change in steam temperature. For example, with the steam generator of the present embodiment, it has been found that when the steam generator is producing 4.0 million BTU/hr it takes a decrease of 0.30 gpm to achieve a water vapor temperature increase from 225° F. to 275° F., or 0.006 gpm/° F., and that half that amount of decrease in water flow is required for producing the same increase in water vapor temperature when the heat output of the steam generator is maintained at 2.0 million BTU/hr.
Accordingly, one needs to know both how much heat is being produced and the difference between the actual and desired temperatures in order to make the changes needed to maintain the desired temperature. Steam generators on the market and used in environments unrelated to creating steam for crop re-hydration offer controls for obtaining stepped steam outputs. The air, fuel and water for each step are manually tuned to achieve a desired steam temperature. The control system opens or closes a set of valves (one for each of the air, fuel, and water) to move to a different output, with each stage set at a predetermined output. U.S. Pat. No. 4,644,967, granted to Wyatt et. on 24 Feb. 1987 is an example of such a control system. This approach has limited functionality when the steam generator is being used to re-hydrate crop, and has cost and reliability problems due to the size and number of valves.
The problem to be solved then is to provide a cost-effective way to control a steam generator, within a relatively short response time, so as to maintain a desired steam temperature.